Maybe/app/models/period.rb

class Period
  attr_reader :name, :date_range

  def self.find_by_name(name)
    INDEX[name]
  end

  def self.names
    INDEX.keys.sort
  end

  def initialize(name: "custom", date_range:)
    @name = name
    @date_range = date_range
  end

  def extend_backward(duration)
    Period.new(name: name + "_extended", date_range: (date_range.first - duration)..date_range.last)
  end

  BUILTIN = [
      new(name: "all", date_range: nil..Date.current),
      new(name: "last_7_days", date_range: 7.days.ago.to_date..Date.current),
      new(name: "last_30_days", date_range: 30.days.ago.to_date..Date.current),
      new(name: "last_365_days", date_range: 365.days.ago.to_date..Date.current)
  ]

  INDEX = BUILTIN.index_by(&:name)

  BUILTIN.each do |period|
    define_singleton_method(period.name) do
      period
    end
  end
end